1. Field of Invention
The present invention relates to catalysts, articles, and methods for treating combustion exhaust gas.
2. Description of Related Art
Combustion of hydrocarbon-based fuel in electrical power stations and in engines produces flue or exhaust gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). But the flue and exhaust gases also contain, in relatively small part, noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of flue and exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of the undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
Typically, flue gases from electrical power stations and exhaust gases lean burn gas engines have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. In such gases, one of the most burdensome components to remove is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The reduction of NOx to N2 is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NO reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Zeolites (i.e., aluminosilicates) having a BEA, MOR, CHA or certain other structures are known to be useful as SCR catalysts. Such zeolites have a molecularly porous crystalline or pseudo-crystalline structure constructed primarily of alumina and silica. The catalytic performance of these zeolites may be improved by incorporating a metal—for example, by a cationic exchange wherein a portion of ionic species existing on the surface of the framework is replaced by metal cations, such Cu2+. Typically, higher metal concentrations correspond to higher catalytic performance.
While the SCR performance of such catalysts is acceptable at normal exhaust gas temperatures of a diesel engine, the catalyst may become exposed to unexpected temperature spikes (e.g., >800° C.) during its useful life. In general, a zeolite's hydrothermal stability can be improved by increasing the zeolite's silica-to-alumina ratio (SAR). However, higher SARs typically limit the amount of metal that can be loaded on the zeolite.
Copper loaded CHA zeolites are known to provide adequate SCR performance provided that the catalyst is not exposed to temperatures in excess of 800° C. However, there remains a need for improved SCR catalysts that are capable of withstanding even higher temperatures with little or no deactivation. The present invention satisfies this need amongst others.